I'd be tempted to stick with the 550 and then go for the tandem. What motor is in the 550? Have you done a fuel consumption/maintenance comparison between the two?
Aside from my tandem, I have an '01 F350 DRW (which is too small) with the old 7.3 because I fear the newer motors. My next go around I'm fairly certain I will be switching to a cabover for a small truck as they are generally: well built, maneuverable, fuel efficient, and dead easy to work on.

Mine has the 6.0 diesel and overall not complaining how it towed the TK. It's just if I move up and get the 75 size with a deckover trailer, I don't think that would be safe or good for the 550. From what I could find the fuel mileage would be very close. I also would like to be able to have an additional 6,000 pounds of carrying capacity.

Those cabovers really don't have a towing capacity?

Quote:

Originally Posted by Danny Boy

Local guy here pulls a 120 sized hoe with a dt 466 single axle, has done for years legally.
I have the same truck and can load a tak 230 track loader and my 161 on ot and it tows great on the flay but is a dog in the hills, always gets there though...

Fortunately it's not hilly here. I also remember guys use to haul their TLB's around with single axles(and alot were gas with 5&2...).